Flat panel displays is a rapidly increasing field. Liquid crystal displays are the most widespread example, and they are currently subject for rapid development.
Transflective liquid crystal displays (LCDs) are characterized by their ability to provide reflective properties, illuminated by ambient light, as well as transmissive properties, illuminated by an internal backlight arrangement. Such displays are advantageous in that they exhibit high readability in bright daylight as well as in dark surroundings.
The transflective property is provided by a transflector that is arranged between the liquid crystal layer and the backlight. The most common transflector is the so-called “hole-in-mirror” type transflector, which is basically a perforated mirror having reflecting portions and transmissive portions (i.e. holes). Transflectors are generally classified by their reflection/transmission ratio, e.g. a 90/10 transflector have 90% reflective surface and 10% transmissive surface.
A transflective display can thus be driven in either of a purely reflective mode, having the backlight turned off, or a transmissive mode, having the backlight turned on. However, ambient light can be exploited even when the backlight is turned on. In such case a combination of transmitted and reflected light is exploited and displayed.
Most transflective liquid crystal displays are full-color displays, and thus feature a color filter. The color filter is typically a patterned RGB filter (Red, Green, and Blue), and the color patterning coincides with a sub-pixel arrangement in the liquid crystal element. RGB filters are additive in that they transmit only one color (R, G, or B) and absorb the remaining colors. Hues are thus formed by combining (adding) light from two or three sub-pixels. Mixing all three colors, of course, forms white light. Due to the absorbing nature of the color filters, brightness is a critical issue in full color displays.
A transflective display is thus arranged with a layer of liquid crystal (the LC layer), a color filter, and a transflector. Ambient light that impinges the display travels through the LC layer and the color filter, is reflected by the transflector (except for the fraction of light that is transmitted through the transflector), and returns back through the LC layer and the color filter. In the transmissive mode, light originates in the backlight and travels first through the transflector (i.e. the fraction of light that is not reflected by the transflector) and subsequently through the LC layer and the color filer. In effect, light originating from the backlight crosses the color filer only once whereas ambient light reflected by the reflector crosses the color filter twice resulting in an increased filtering effect and thus in a more saturated color.
Consequently, the color saturation in a transflective display typically differs between the transmissive mode and the reflective mode. This problem is further amplified by the fact that the color spectrum of the ambient light that is exploited in the reflective mode typically differs from the color spectrum of the backlight that is exploited in the transmissive mode. The resulting color differences make smooth transitions between the drive modes difficult or even impossible. Furthermore, the white point (i.e. the perceived color and intensity of the whitest possible light) typically differs between the transmissive and the reflective mode.
The color-balancing problem is further complicated by the fact that the sub-pixels have limited contrast. In other words, it is not possible to turn the sub-pixels off completely. The limited contrast thus deteriorates the color saturation, since, for example, the most reddish hue will still be deteriorated by a certain amount of green and blue light.
Consequently, the color saturation and white points in the transflective and reflective drive modes need to be improved as well as balanced.
One approach for alleviating this problem is to use a color filter that does not fully cover the reflective portions of the transflector (a so-called pin-hole color filter). Such a solution is indicated in e.g. EP1279996A2. In effect, in each sub-pixel, the reflected light consists of a filtered part that has crossed the color filter twice, and an unfiltered part that has not crossed the color filter at all. The addition of unfiltered light to the reflected light reduces the saturation, and by suitable choice of color filter coverage, the saturation can be balanced with that of the transmitted light.
Another approach is to use a color filter that has differentiated thickness for the transmissive and reflective portions (a so-called MT structure). To put it simple, the color filter thickness may be doubled in the areas that correspond to transmissive portions of the transflector, whereby the same distance is traveled by ambient light and by light originating from the backlight. Such as solution is described in, for example, EP1279966.
Still one measure that can be employed in order to balance the white points in the transmissive and reflective modes is use different reflection/transmission ratios for the different colored sub-pixels. For example, in case the transmissive white point is slightly yellowish and the reflective white point is slightly bluish, the reflection/transmission ratio for the blue sub-pixel might be reduced (i.e. increasing the transmissive portion).
However, balancing and improving the white points on one hand and balancing and improving the color saturations on the other hand are two different, but indeed interlinked problems. Thus, optimizing one feature is likely to deteriorate other features. Complete optimization of the overall display performance therefore requires many degrees of freedom in the form of independent parameters that can be tuned freely.
The measures currently available do not meet this requirement, and it is therefore an object of the present invention to provide further measures for optimizing and balancing the color saturation and white points in transflective liquid crystal displays. In addition, as a general requirement, the optimization must be feasible without involving any substantial extra costs.